UltraHDR Loader needs to support ISO 21496-1 gainmap metadata

[cmahnke]

Description

The UltraHDR format has been evolving: While it adopted gain map encoding based on the Adobe XMP specification (this method embeds a gain map together with XMP in a JPEG file to enable HDR viewing while retaining SDR compatibility). But to resolve cross-platform fragmentation, Google and Apple are now adopting the unified ISO 21496-1 standard in newer OS versions like Android 15 and iOS 18.

This isn't reflected yet in the UltraHDR loader, it checks for the XMP metadata and if those aren't found, it bails out...

Solution

Support the ISO 21496-1 encoding of gain maps in UltraHDRLoader

See also the README of libultrahdr, the XMP support is already disabled as default while building.

Alternatives

Reencode images.

While this might work for some, the toolchain is not exactly easy to set up.

Solving this issue would have the benefit to be able to use HDR textures directly taken with a (iOS) mobile phone

Additional context

No response